1. Conclusions Embryo-larval bioassays are very sensitive to acutely-toxic compounds mobilized into the water column. The most toxic fraction of the fuel was mobilized to the water column. In contrast the fraction of weathered fuel accumulated in the sand as solid particles and tar balls, visually much more conspicuous, lacked toxicity to water column organisms. ABSTRACT An evaluation of the toxicity of seawater and sand sampled from an area of the Galician coast and shelf after the Prestige oil spill was attempted by using the bivalve and sea-urchin embryogenesis bioassay. Coastal water samples were frozen and toxicity testing was delayed until the reproductive season of the sea-urchin Paracentrotus lividus. Sand samples were elutriated and tested within a maximum of 13 d from sampling, using bivalve embryos of Venerupis pullastra and Crassostrea gigas obtained from commercial stocks. Coastal sand elutriates were in general non toxic for embryos, not even during the first weeks after the fuel oil spill. In contrast, high toxicity was found in the seawater during the first days after the fuel oil spill in the most impacted site, with complete inhibition of embryogenesis even after 4-fold dilution with control water. In a lower degree toxicity persisted for two months in light-exposed coastal water from the most impacted site. These findings stress the impact to aquatic organisms of the less conspicuous water-accommodated fraction, rather than the more visible oil slick, in an oil spill event. In the shelf samples the embryogenesis success reached high values (80-88%, with 86% in the control), indicating a lack of toxicity in the sediments, disregarding organic content (ranging from 0.2 to 2.0%) or percentage of fine fraction (from 2 to 48%). This lack of toxicity agrees with the PAH concentrations measured in replicates of the same sediments. These data support the conclusion that the spatial dispersion of the fuel reaching the seafloor avoided a high impact on the benthic communities of the Galician shelf. Evaluation of the toxicity of seawater and sandy sediments affected by the Prestige fuel-oil spill by using the marine invertebrate embryogenesis bioassay Saco-Álvarez, L., Beiras, R. Laboratorio de Ecoloxía Mariña, Facultade de Ciencias do Mar, Universidade de Vigo, E-36310, Galicia. Email: lilisaco@uvigo.es (a)(b) Fig. 3. Paracentrotus lividus. Length of pluteus larvae (mean  VC, n=5) obtained after 48 h dark (a) and light (b) incubation of fertilized eggs in undiluted surface water from M1 and M2 periodically sampled from Nov 2002 to Apr 2003. ** and *** indicate 99 % and 99,9 % confidence significant reduction of larval size. Sea water bioassays Larval length Sea water Frozen until Reproductive period Sea water 20ºC >30‰ Incubation 20 ºC - 48 h darkness 14h light:10h dark Fertilization Paracentrotus lividus 20 eggs/ml Sampling sites Sediment bioassays Fig. 1. Venerupis pullastra. Percentage of normal larvae (mean  SD, n=5) obtained after 48 h incubation of fertilized eggs in different dilutions of M1 and M2 sand elutriates sampled Feb 5 th in the Galician coast. * indicates 95 % confidence significant reduction of embryogenesis success. Fig. 2. Venerupis pullastra. Percentage of normal larvae (mean  SD) obtained after 48 h incubation of fertilized eggs in sediment elutriates sampled in the Galician shelf. Elutriation (30 min) Elutriate 400 ml sea water + 100 g sediment Normal D-larvae Incubation Dilution Fertilization 20 eggs/ml or Venerupis pullastraCrassostrea gigas M1: Furnas M2: Vilar M3: Aguiño St 21 St 17 St C St 13 St 5 Coastal SedimentsShelf Sediments 20 ºC - 48 h